This invention relates generally to inspection of nuclear reactors, and more particularly to an electric discharge machining (EDM) apparatus for obtaining a material sample within a nuclear reactor pressure vessel.
A reactor pressure vessel (RPV) of a boiling water reactor (BWR) typically has a generally cylindrical shape and is closed at both ends, e.g., by a bottom head and a removable top head. A top guide typically is spaced above a core plate within the RPV. A core shroud, or shroud, typically surrounds the core and is supported by a shroud support structure. Particularly, the shroud has a generally cylindrical shape and surrounds both the core plate and the top guide. There is a space or annulus located between the cylindrical reactor pressure vessel and the cylindrically shaped shroud.
Internal structures of operating BWRs are susceptible to various corrosive and cracking processes. Stress corrosion cracking (SCC) is one known phenomenon occurring in reactor components, such as structural members, piping, fasteners, and welds, exposed to high temperature water. The reactor components are subject to a variety of stresses associated with, for example, differences in thermal expansion, the operating pressure needed for the containment of the reactor cooling water, and other sources such as residual stresses from welding, cold working and other inhomogeneous metal treatments. In addition, water chemistry, welding, heat treatment and radiation can increase the susceptibility of metal in a component to SCC.
Surface treatments such as Noble Metal Chemical Addition (NMCA) are used to produce an adherent, micro-layer coating or film on the surface of nuclear reactor components. During application, the NMCA treatment may not be uniformly deposited, and during service, the treatment may be removed from the surface due to corrosion or other mechanisms. To measure the NMCA distribution or concentration on the surface, a sample of the material surface oxide is needed. A sample of sufficient surface area and mass needs to be obtained with minimal damage to the reactor component.
One method used to obtain a sample of the material surface oxide involves the use of a small grinding wheel or stone and an associated vacuum to collect the grindings or residue. This method typically does not provide a suitable sample size for the necessary measurements. Another method to obtain a sample of the material surface oxide involves the use of a machining burr or similar tool to remove chips of the surface metal that contain the surface oxide. This method causes a large surface indentation, and may also destroy or disperse the oxide of interest before the metal chips can be removed. Still another method of obtaining a sample of the material surface oxide involves removing a large sample of material from the reactor component by machining or a similar process. The removal of a large sample significantly alters the surface of the reactor component and may require repair, special analysis of the reactor component, or future in-service monitoring of the reactor component.
In one aspect, an electric discharge machining sampling apparatus for obtaining samples from a surface of metal components is provided. The sampling apparatus is operable underwater and includes a base plate and an electrode assembly movably coupled to the base plate. The electrode assembly includes an electric discharge electrode and an electrode holder with the electrode including at least one bore extending therethrough. The sampling apparatus also includes a particle collection assembly operatively coupled to the electrode. Each electrode bore is in flow communication with the particle collection assembly.
In another aspect, an electric discharge machining sampling apparatus for obtaining samples from a surface of components in a nuclear reactor is provided. The sampling apparatus is operable underwater and includes a base plate and an electrode assembly movably coupled to the base plate. The electrode assembly includes an electric discharge electrode and an electrode holder with the electrode including at least one bore extending therethrough. The sampling apparatus also includes a particle collection assembly operatively coupled to the electrode. Each electrode bore is in flow communication with the particle collection assembly. The particle collection assembly includes a filter media positioned in a filter housing with the filter housing operatively coupled to a vacuum source.
In another aspect, a method of performing in-situ sampling of metal surfaces of components in a nuclear reactor is provided. The method includes positioning an electric discharge machining sampling apparatus adjacent a metal surface of a reactor component in the reactor, activating the sampling apparatus to produce a debris of particles from the surface of the reactor component, and collecting the particle debris. The sampling apparatus is operable underwater and includes a base plate and an electrode assembly movably coupled to the base plate. The electrode assembly includes an electric discharge electrode and an electrode holder with the electrode including at least one bore extending therethrough. The sampling apparatus also includes a particle collection assembly operatively coupled to the electrode. Each electrode bore is in flow communication with the particle collection assembly.